Since utility costs are a primary factor in the design and operation of plants in the chemical process industries, it is established practice to minimize energy consumption by recovering energy from process or utility gas streams and then recycling the recovered energy to the process (or else using the recovered energy for another purpose). The recovery of energy from a gas stream is achieved by heat exchange with a selected fluid heat sink, e.g., a cooler liquid or gas stream. A typical heat recovery and utilization technique involves heat exchange between a hot reactor effluent gas and water so as to convert the latter to steam, and using the steam as a vehicle for recycling the recovered heat to the reactor.
It is a common occurrence in a chemical plant to have a process or utility gas stream that is at a relatively low temperature but contains more than an insignificant percentage of the energy generated in or supplied to a process. It is desirable to treat the low temperature process or utility gas stream to recover a substantial portion of its energy content. Frequently, however, the energy content is treated as waste heat because its recovery is uneconomical or can be achieved only with difficulty or substantial inconvenience. In this connection, it is to be appreciated that the lower the temperature of the process or utility gas stream, the lower the temperature of the fluid heat sink must be in order for the heat sink to adequately recover energy from the gas stream. One of the problems incurred in attempting to recover energy from a low temperature gas using a liquid heat sink such as water for heat recovery is that the temperature of the heat source, i.e., the process gas stream, may be so low that the liquid heat sink will not boil at a temperature lower than the heat source unless its pressure is lowered. However, if the heated heat sink should constitute a material that is to be fed to another stage of the process operating at a higher pressure, reducing the pressure of the liquid heat sink to enable it to be vaporized by boiling may be undesirable or too costly, since energy must be consumed in compressing the heat sink vapor to the higher pressure desired for the subsequent stage.